Japanese Patent Laying-Open No. 2013-185507 (patent document 1) discloses a condition monitoring system (CMS) which monitors a condition of a mechanical element of a wind turbine. This condition monitoring system takes in a signal of a vibration sensor provided to the mechanical element, records over a long period of time how a quantity of a condition representing a state of vibration in a rated operation (hereafter referred to as a diagnosis parameter) varies with time, and based on the diagnosis parameter's increase rate, a feature of the variation and the like, determines whether the mechanical element has a failure (see patent document 1).
Furthermore, a wind turbine generally includes a nacelle provided on top of a tower and a yaw bearing which supports the nacelle on top of the tower turnably. The yaw bearing is provided at a high place and generally has a large size, and accordingly it is not easy to replace it with another. Japanese Patent Laying-Open No. 2012-122342 (patent document 2) describes a wind turbine allowing a simple device to be used to replace a yaw bearing with another (see patent document 2).